Head
and body length of S. sciureus is 260-360 mm, tail length
is 350-425 mm, weight ranges from 750-1,100 grams. The short,
thick, soft fur is brightly colored and there is a circular
patch around the lips and nostrils which is hairless. Coloration
varies, but most commonly the markings are as follows: white
around the eyes extending behind the ears, down the sides of
the throat and neck; gray to black on the top of the head, reddish
or yellow forearms, hands and feet with the shoulders and hind
feet touched with gray; underparts white to ochre; and the tail
colored similarly to the body, but with the terminal portion
black. Coloration and markings are similar in all ages and both
sexes. The large eyes are closely set and the ears are shaped
like human ears. S. sciureus has a short but well-developed
thumb and a long tail which is not prehensile.

Common Squirrel Monkeys are found in both primary and secondary
forests and cultivated areas preferring habitats with streams.
They are active in day and night. They are considered both arboreal
and diurnal animals. The species feeds on insects found in its
habitat. However, the Common
Squirrel Monkey is not an absolute insectivore. They are also
frugivorous animals. Their diet includes various fruits and
berries. It has been established that sometimes they follow
Cebus monkeys to find fruits. It is interesting to know that
Common
Squirrel Monkeys can also feed on molluscs, frogs, and small
vertebrates. They often inhabit areas near water and yet they
can obtain most of their water from their meals. Sometimes,
Squirrel Monkeys drink water from tree holes and on the ground.

They
do appear to be particularly territorial. This primate forms
larger groups than any other New World Monkeys. Social subgroups
are often comprised of pregnant females, females with young,
and adult males. The mating season reveals and hierarchy achieved
by fierce fighting, and the dominant males interacting with
the females. Once the females give birth they form a subgroup
which excludes the males and likely has a hierarchy of its own.
Another subgroup of young males and preadult males, has individuals
who leave to join the adult males as they reach sexual maturity,
sometimes being required to fight to win a place in this group.
S. sciureus is among the most vocal of all primates using
chirps and squeaks for alarm, squawks and purrs during mating
and birth seasons, barks of aggression and screams when in pain.
26 distinct calls have been recorded in these monkeys.

Reproductive data is available for a captive population and
indicates a 2-4 month mating period during the dry season. Births
occurred between late June and early August, gestation lasting
from 152-172 days. At birth newborns weigh about 100 grams and
cling to their mothers back for the first few weeks. At 1 year
of age they are independent, females reach sexual maturity at
3 years and males at about 5.
S. sciureus is found in N Brazil north of the Amazon-Jurua
system, and south of the Amazon, east of the Rio Xingu or the
Rio Iriri; Marajo Isl (Brazil), Guyana, French Guiana, Surinam,
Venezuela, Columbia, E Ecuador, and NE Peru.